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New hominin species reported from Ethiopia

Hominin remains comprising a complete lower jawbone, a partial lower jawbone and two partial upper jawbones, together with some accompanying teeth have been described as a new species, Australopithecus deyiremeda. The fossils were excavated in 2011 in the Woranso–Mille study area, central Afar, Ethiopia. They were found in deposits dated from 3.3 to 3.5 million years old, making Australopithecus deyiremeda a contemporary of Australopithecus afarensis (the species to which the well-known fossil ‘Lucy’ belongs) and the controversial hominin species Kenyanthropus platyops. The specific name deyiremeda means ‘close relative’ in the local Afar language and follows a now-established tradition of using local languages to name hominin species.

Australopithecus deyiremeda is distinguished from Ardipithecus ramidus by its thicker dental enamel and more robust lower jawbone. It is distinguished from Australopithecus afarensis by a number of features of its lower jawbone, by the positioning of its cheekbones in relation to the upper jawbone, and by its smaller back teeth.

What are the implications of this discovery? For a long time, it was believed that there was just the one hominin species, Australopithecus afarensis, living in the period from four to three million years ago, in East Africa. It was possible to argue that the earlier Australopithecus anamensis (4.2 to 3.9 million years ago) and the later Australopithecus garhi (2.5 million years ago) were simply early and late forms of the same species and that Australopithecus bahrelghazali from Chad (known from a single 3.5-million-year-old specimen) represented a Central African extension of its range. On this view, Australopithecus afarensis was a single, long-lived, geographically widespread species, capable of occupying a wide range of habitats. Not until 2.8 million years ago did other hominin species start to appear: Australopithecus africanus and later Australopithecus sediba in South Africa and the so-called robust australopithecines (Paranthropus) in both South Africa and East Africa.

Even if Kenyanthropus platyops is rejected, this view is no longer tenable. There is now incontrovertible evidence that multiple australopithecine species were living in East Africa during the Middle Pliocene. It is also notable that Australopithecus afarensis has been recorded at Hadar, only 35 km (20 miles) north of Woranso–Mille. Not only did these species overlap in time, they were close in geographical terms, probably occupying differing feeding niches.

Early hominin evolution has been described as more of a tangled bush than a family tree. In addition to Australopithecus afarensis, Australopithecus deyiremeda and possibly Kenyanthropus platyops, it is likely that the Ardipithecus line was still in existence at this time. The Woranso–Mille site has also yielded a 3.4-million-year-old partial hominin foot with an opposable big toe. Though it has not been assigned to a particular species, the toe suggests Ardipithecus or something very similar.

With the LD-50-1 lower jawbone pushing back the origins of Homo to 2.8 million years ago, later australopithecines such as Australopithecus sediba have been bumped from the list of possible human ancestors. However, the Woranso–Mille discovery means that we are no nearer identifying from just which part of the ‘tangled bush’ the first humans emerged.